An organic semiconductor is expected to be applied to a photoelectric conversion element such as an organic thin-film solar cell. For example, by using the organic semiconductor as a p-type semiconductor material, it becomes possible to apply an inexpensive coating method to formation of an active layer. On the other hand, in view of energy demand and CO2 emission reduction, a solar cell is expected as one of clean energy having a small environmental load, and its demand is rapidly increasing. Though a silicon-based solar cell is a mainstream at present, cost curtailment thereof is difficult. As an inexpensive solar cell, a CdTe type solar cell is also known, but Cd being a harmful element is used, there is an apprehension that an environmental problem occurs. Under the circumstances, development of the organic thin-film solar cell is expected as a next-generation solar cell which is low in cost and harmless.
In order to heighten practicability of the organic thin-film solar cell, improvement of a power generation efficiency of the organic thin-film solar cell is strongly demanded. As a measure to improve the efficiency of the organic thin-film solar cell, it is known to apply a bulk hetero junction structure in which a mixture of an n-type semiconductor material and a p-type semiconductor material is used to a photoelectric conversion layer. For example, fullerene or its dielectric is used for the n-type semiconductor material and an organic semiconductor such as polythiophene and its dielectric is used for the p-type semiconductor material. Further, a photoelectric conversion efficiency of an organic thin-film solar cell is heightened by interposing an electron transport layer or a hole transport layer between a photoelectric conversion layer having a bulk hetero junction structure and an anode or a cathode, thereby to selectively flow an electron or a hole obtained in the photoelectric conversion layer to each pole.
A sufficient photoelectric conversion efficiency cannot be necessarily obtained only by the efficiency improvement measure of the organic thin-film solar cell as described above. Thus, it is proposed to interpose a self-assembled monolayer between an electron transport layer and a photoelectric conversion film. It is reported that introduction of the self-assembled monolayer shifts an energy level between an electrode and the photoelectric conversion layer, whereby a performance of an organic thin-film solar cell is improved. The self-assembled monolayer interposed between the electron transport layer and the photoelectric conversion film is required to be excellent itself in electron-accepting property and to be able to be formed uniformly and densely on a surface of the electron transport layer. However, a conventional self-assembled monolayer material cannot satisfy such required properties sufficiently. Thus, there are required a self-assembled monolayer material capable of improving a photoelectric conversion efficiency and a solar cell using the same.